The present invention relates to an electronically controlled power steering system.
JP 53-32129 B discloses an electronically controlled power steering system which includes sensors for measuring a vehicle speed, a side acceleration and a steering wheel angle, an arithmetic circuit making a judgement on the sensor output signals, and a servo valve converting an output of the arithmetic circuit to a hydraulic fluid pressure signal which is supplied to a reaction chamber of a servo mechanism of a steering linkage. With this known power steering system, a steering torque needed to manipulate a steering wheel increases in proportion to an increase in a steering wheel angle since a flow rate of hydraulic fluid supplied to the servo mechanism decreases as the steering wheel angle increases.
JP 57-209471 A discloses an electronically controlled power steering system which includes a steer angle sensor, a vehicle speed sensor and a control circuit receiving the sensor output signals, a solenoid operated bypass valve fluidly disposed between both chamber of a power cylinder, and a driver responsive to the output of the control circuit for regulating electric current passing through the solenoid operated bypass valve such that the flow rate of bypass fluid therethrough varies in response to the magnitude of the electric current. With this known power steering control system, a steering torque needed to manipulate a steering wheel increases as the steer angle increases and/or the vehicle speed increases.
The common problem encountered with the above-mentioned power steering systems resides in that a steering torque needed to manipulate a steering wheel increases excessively in turning the steering wheel through a large steering wheel angle, and thus controllability of a vehicle is very bad.
An object of the present invention is to provide an electronically controlled power steering system which restrains an increasing trend of a steering torque during turning a steering wheel through a large steering wheel angle so as to make it easy to control a vehicle during turning of the steering wheel through the large steering wheel angle, thus providing enhanced running stability and safety.